Why the lens? The one you have is superlative. As for 5D2 --> 3, now is the time, before 5d3 loses a rebate tomorrow and 5d2 prices keep falling... The question, as always, is "is the 5d2 limiting you in any way the 5d3 will improve on?" Could be any number of reasons: AF, low-light/high-ISO performance, you want the FPS, or in-camera-HDR... But if you're happy with those things on your 5d2, then just keep enjoying it!

AF and the high ISO performance would be the main reasons. Actually another reason is although I absolutely love my 70-200 2.8, I kin of get sick of the size of it. I think I'd really enjoy the size of the f/4 IS and I thought the high ISO of the mark 3 could compensate for the loss if 2.8... Is my logic flawed or am I not thinking about something? P.s. either way, I'll probably end up getting either the canon 85 f/1.8 or the 135 f/2 once I nail down the focal length I'm happier with--if that adds anything to the decision

Haha- my 85/1.8 is on its way now. Good choice! ;-) (that said, I'd have gone for 135 if I had FF, it's just longer than I need on APS-C)

Anyway, I digress. I've not used either of those particular 70-200 lenses, because I don't typically find myself needing IS at those lengths (usually using them for sports, high shutter speeds). But everything I've seen with the 2.8 IS ii is cut-you sharp. Not that the f/4 is a bad lens, but the 2.8 ii is just so sharp, and fast to AF, and doesn't degrade as much with extenders... So while you are correct, you'll be able to use higher ISO to compensate for the stop in aperture, but you'll lose the DOF control, and I dare say a bit of IQ from your lens... Just my impressions.

Yeah I would have to agree with above. Keep the lens. It's worth it. I have both mark ii and mark iii of the 5d and the iii is a epic camera love shooting it. Make the ii feel a little relic like. Picture quality isn't dramatic but the camera is worth the upgrade. The downgrade on the lens would just limit your mark iii and you will miss it.

The 2.8ii is a beast. Hence the reason I went for the f4is. My copy is cut your teeth sharp so I feel no need to upgrade especially since I get such great ISO performance out of the 5diii. Sure I may get another 5-10% iq with the 2.8 but its not worth IMHO the weight difference. I can shoot all day with the f4 is and it feels like nothing hanging off my shoulder.

You did not say what you shoot so I am not sure what people on this forum can share with you that would be meaningful. The 70-200 II is a great lens. the 5D2 is a great camera except for the AF... So do share what you shoot so that we can offer more meaningful suggestions.

I do a few weddings here and there but mostly portraits, and travel photography.

I'd suggest you retain your current setup and save up for the 5d3. For what you shoot I would consider the 70-200 2.8 II exceptionally useful ... Selling this lens for the f/4 may turn out to be a decision you end up regretting.

Also IMHO for what you shoot I don't think that the AF of the 5d3 is essential ... It would be great to have it but you really shouldn't be selling the lens for that. The 70-200 2.8 II will AF better on the 5d3 (you can use the dual cross type AF points) as compared to the 70-200 f/4.